1. Field of the Invention
The invention relates generally to ion sources for mass spectrometers, and, more particularly, to carbon nanotube-based ion sources for mass spectrometers.
2. Background Art
Mass spectrometers are powerful instruments for the analysis of a wide variety of samples. In order to perform mass analysis, the samples need to be vaporized. The gas molecules are then ionized by an ion source. An efficient ion source will convert as many sample molecules into ions as possible and produce an optimal beam for the specific type of analyzer. The most common ion source is the electron ionization (EI) source. In an EI source, electrons are produced by thermal emission from a hot filament, which is heated by a current flowing through it, located outside the ionization chamber. The electrons are accelerated by an electric field to a desired level of energy. This energy level is typically round 70 eV, but can vary from about 10 eV to upwards of 150 eV, as defined by the potential difference between the filament and the ionization chamber. When the electrons collide with sample gas molecules in the ionization chamber, the gas molecules each lose an electron and become positively charged. Once the sample molecules acquire positive charges, they can be accelerated out of the ionization chamber and guided into the entrance of the mass spectrometer by an applied electrostatic field.
While various configurations have been developed for EI sources used in mass spectrometers, the configuration originally design by Nier and the variants thereof are the most common. FIG. 1 shows two views of a basic Nier design ion source that uses a hot wire filament 10 to produce an electron beam 19; one view (1A) is perpendicular to the xz plane, while the other view (1B) is in the xy plane, where the x-axis is the direction of motion of ions leaving the ion source and the y-axis is in the direction of mass separation and the z-axis is perpendicular to both the x- and y-axes. The electron beam 19 is typically accelerated to about 70 eV of energy. The electron beam 19 is designed to interact with molecules introduced into the ionization chamber 11, under high vacuum. The interactions produce molecular ions and fragment ions that can be accelerated out of the ionization chamber 11.
Because the electron beam is somewhat divergent, a pair of permanent magnets 14 is added to force the electron beam 19 to travel in a spiral path, which constrains the motion of the electrons to a narrow beam. Any component of electron motion which is perpendicular to the magnetic flux acts to deflect the electrons into a spiral trajectory. This has the effect of increasing the probability of the interactions between the electron beam 19 and the molecules in the ionization chamber 11 in the region where they are extracted as positive ions. In this way good sensitivity and resolution (low ion energy spread) are achieved.
Once ionized, the newly charged particles are repelled by the ion repeller 12 to move towards an exit of the ionization chamber 11. In addition, the charged particles are accelerated by the accelerating potential 15, focused by the focusing half plate 16, and filtered by the alpha slit 17 to form a focused ion beam 18. The focused ion beam 18 is then introduced into a mass filter (not shown), where they are separated according to their mass-to-charge ratios.
Interactions between the sample gas and the hot filament may result in changes in the electron work function of the filament. In order to provide a constant intensity of the electron beam 19, an electron trap 13 is typically provided in an EI source. The electron trap 13 is to capture the proportion of the electron beam 19 that exits the ionization chamber 11. In addition, the electron trap 13 may also be used to monitor the intensity of the electron beam 19 in order to provide a feedback control to the current flowing through the filament 10. The feedback control enables the filament 10 to produce a constant intensity electron beam 19 as measured at the electron trap 13.
In a typical EI source, the filament 10 is a wire and made of a refractory metal. The current heats the filament 10 to a temperature (about 2000° C.) at which thermionic emission of electrons occurs. The filament 10 is typically held at a negative electric field relative to the ionization chamber 11 (e.g., by applying an potential difference across the filament 10 and the ionization 11) so that the emitted electrons are accelerated from the hot filament 10 in the direction of the gradient of the electric field. The translational energy of the electron beams affects the nature of the interactions between the gaseous sample molecules and the electrons.
Although a typical ion source design is based upon well established principles, the performance of an ion source depends upon the interactions of many subtle design characteristics. There are several problems associated with the filament assemblies used in electron impact or chemical ionization source. The primary problem is that the origin and trajectory of the electrons are ill defined. Additionally, the electron emission relies on the vaporization of material, which results in a limited filament lifetime. Interactions between the sample gas and the hot filament may result in changes in the electron work function of the filament. As noted above, a trap electrode (shown as 13 in FIG. 1) may be used in a feedback circuit to regulate the electron beam 19 intensity. However, regulation of the trap current will alter filament temperature. This can lead to fluctuation in the temperature distribution in the ion source and cause the assembly to become misaligned. These effects lead to changes in absolute sensitivity, relative sensitivity, and the degree of molecular fragmentation. As a result, it is often difficult, if not impossible, to de-convolute a mass spectrum of a complex mixture sample, due to inevitable uncertainties in the contributions from the components in the mixture.
Thus, to avoid mass analysis complications, it is desirable to have an ion source that can produce a stable stream of electrons with predictable trajectories and uniform density.